This invention relates to a bipolar separator assembly and, in particular, to a bipolar separator for use in molten carbonate fuel cells.
A fuel cell is a device that directly converts chemical energy in the form of a fuel into electrical energy by way of an electrochemical reaction. In general, like a battery, a fuel cell includes a negative electrode or anode and a positive electrode or cathode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. In order to produce a useful amount of power, individual fuel cells are typically arranged in stacked relationship in series.
In molten carbonate fuel cells, an electrically conductive bipolar separator is used to separate adjacent cells in a fuel cell stack. The main functions of a bipolar separator of molten carbonate fuel cells are to separate cathode gas from the anode gas and to provide current transmission and load pressure distribution in high temperature corrosive environment. A typical bipolar separator comprises a thin flat metal plate member, or separator sheet, having first and second opposing surfaces which form active areas and which are compatible with oxidant and fuel gases, respectively. First and second opposing ends of these areas are folded upward and toward each other to form first and second pockets. Third and fourth opposing ends of the areas are folded downward and toward each other to form third and fourth pockets. The first and second pockets support a fuel cell matrix on the first surface side of the plate member, while third and fourth pockets support a fuel cell matrix on the opposing second surface side of the plate member. In this way, the first, second, third and fourth pockets form wet seals for the molten carbonate fuel cells that seal both the separator sheet and the matrix and create flow fields for the fuel and oxidant gases.
In order to realize long-term stability of the fuel cell, the wet seals and edges of the separator sheet must be kept corrosion free. In addition, corner areas of an externally manifolded fuel cell stack are critical for sealing to the manifold and thus, typically need to be as flat as possible. U.S. Pat. No. 6,372,374, assigned to the same assignee herein, discloses a bipolar separator with improved wet seals in which the wet seals are made separately from the separator sheet and are then welded to the separator sheet during the assembly process. In the configuration disclosed in the '374 patent, the anode and cathode current collectors extend into the pockets formed by the attached wet seals, which provide uniform support to the electrodes and the matrix. These features of the bipolar separator are important to the performance of the fuel cell system.
When the bipolar separator of the '374 patent is manufactured and assembled, the five components of the separator, i.e. one plate member and four wet seal pockets, have to be welded together. The welding of these components results in a weld around the entire perimeter of the bipolar separator, which in typical fuel cells is about 160″ of weld. In addition, portions of this weld extend in the corner areas of the fuel cell stack, which, as mentioned above, are critical for sealing and supporting the external manifolds against the fuel cell stack. However, the welding process is difficult and costly. In addition, to obtain consistent results, it is necessary to weld the wet seal pockets to the plate member before the corrugated current collectors are in place which allows for properly fixturing with chill-blocks. Since the corrugated current collectors extend under the wet seal pockets, the current collectors need to be deformed and manipulated in order to slide them into place. However, this process requires several people to perform and is very difficult to automate effectively.
Therefore, it is an object of the present invention to provide a bipolar separator assembly which includes wet seal pockets made separately from the plate member but which does not require welding around the entire perimeter of the bipolar separator in order to attach the wet seal pockets to the plate member.
In addition, it is a further object of the present invention to eliminate manual manipulation and installation of the corrugated current collectors into the bipolar separator, and in particular, into the areas under the wet seal pockets of the bipolar separator.